The lumber industry over the past century has been tending to sort lumber into various groups referred to as grades. The graded lumber has more value than ungraded lumber. During the last few years, an electromechanical grading system has been developed for sorting lumber according to its modulus of elasticity (bending stiffness). This system, known as machine stress rating (MSR), was implemented in North America in the early 1960's. Background information on MSR sorting methods and equipment can be found in U.S. Pat. No. 3,194,063 (McKean) and 3,196,672 (Keller). A machine known as the CLT-Continuous Lumber Tester, hereinafter referred to as CLT, is based on the Keller patent and is responsible for most of the MSR lumber production in high speed North American lumber mills. Information about the CLT as it is presently manufactured and sold can be found in a brochure entitled CLT-Continuous Lumber Tester by Irvington Moore and Metriguard Inc., the joint producers of this equipment.
In the production-line the CLT measures the modulus of elasticity, hereinafter referred to as E, along the length of each board. The CLT places the board into an E category and automatically marks it based on two numbers determined by the CLT. These numbers, average E and low point E, are respectively the average and the lowest of the E measurements along the length of the board, and the E category of the board is identified from a comparison of average E and low point E with threshold values. When compared with visually graded lumber, MSR lumber has greater value in engineered structures where knowledge of material properties is important.
In Europe, Australia, and New Zealand, the practice for MSR lumber production requires that lumber be marked according to E category on a local basis along the length of each board. This requires the capability of a rapidly changing mark along the length of each board as well as from board to board. Additionally, in North America and elsewhere, knowledge of the E variation along the length of individual boards of lumber is becoming more important. Low E sections can be removed and high E sections can be joined together by existing finger end-joining technology, thereby producing joined boards having high E throughout. Studies of laminated beam properties, which utilize maps of local E values, will benefit from the efficient and precise E category marking on the constituent laminae.
Most of the MSR lumber produced in North America has an ink spray mark applied along a short length at a single location on each board, the mark being applicable to the entire board. The E category is identified by the spray color, but the shape and location of the mark are not critical other than the requirement of it being visible to either a machine or a human as input for use in determining the board grade and for subsequent sorting operations.
Recent introduction of the CLT for MSR lumber production in Australia and New Zealand has required a high speed ink marking system for categorization of E on a local basis rather than on a board by board basis as in North America. Spray marking technology already in use in Australia and New Zealand prior to the CLT's introduction was not satisfactory for use with the CLT because of the CLT's much higher operating speeds than the equipment it replaces.
Lumber speeds in the CLT can be as much as 24 feet/second (7.3 meters/second). At 24 feet/second, the lumber moves past a fixed spray marking nozzle at the rate of 1 inch (2.54 cm) in 0.0035 seconds. Consequently, to achieve a marking resolution in the neighborhood of 1 inch, the marking equipment must be capable of turning on and off in about 0.003 to 0.005 seconds.
For use with the CLT in Australia and New Zealand, Metriguard Inc. developed a high speed ink marking system having the required response time. The system included a grade spray controller for use in identifying E category on a local scale, an ink tank system, high speed solenoid valves for controlling ink flow (ink valves), and a nozzle block arrangement configured to spray 5 different ink colors. Ink valves used were the Model AM-055-1-12 available from Angar Scientific Company, Inc. of Cedar Knolls NJ. The nozzle block is manufactured from a block of stainless steel and has 5 ink inlet ports, 1 inlet air port, 5 exit orifices, and the required internal ink and air passageways. These internal ink and air passageways are drilled deeply using a small diameter drill bit. It is difficult to perform this drilling operation and in the event of a drill bit failure the block is rendered useless and must be scrapped. The nozzle block is configured for direct mounting of 5 ink valves such that the valves control ink flow in the corresponding nozzle block ink passageways. A continuous flow of air is routed through the exit orifices as a transport medium to carry ink to the lumber. Ink flow exiting an ink valve travels via an internal passageway in the nozzle block and joins an internal air passageway leading to a corresponding exit orifice. The ink joins with the air flow within the nozzle block, and the mixture is carried out of the exit orifice by the continuously flowing air. Additional information about the Metriguard high speed ink marking system is contained in the Metriguard Inc. CLT Grade Spray Controller Operation Manual.
The Metriguard high speed ink marking system has been successful at CLT speeds; however, it requires careful control of both air pressure and ink pressure to achieve the precision of marking desired at these speeds. The nozzles can produce a fog instead of a spray. The fogging is believed caused by the air meeting the ink internally in the nozzle so that the turbulent action of the mixture in the remaining internal passageway prior to the exit orifice atomizes the ink into a finer spray than is desireable for a crisp ink mark. Another problem is that the ink marks can be elongated after the ink valves are shut off. It is believed that the mark elongation is caused by residual ink adhering to the internal walls of the nozzle block in the region downstream of the point where the air and the ink come together and are mixed. When an ink valve is shut off, the continuous air flow carries out the residual ink, leading to elongation of the ink mark past the point corresponding to the ink valve being shut off. Still another problem is that for some combinations of air and ink pressures, ink can flow backwards into the air line, or air can flow into the ink line. By trial and error, ink and air pressures can be set to minimize these problems, but a less critical solution was desired.
High resolution ink marking involves low transit time of ink in flight to the moving lumber target. Consequently, high ink velocities from the exit orifice are desired, and this is achieved at significantly lower pressures when using a gas such as air for a transport medium compared with the alternative of using just ink alone. The conclusion is that a system using a gas to carry ink to the target is desirable.
Introducing ink to an air stream external from the nozzle rather than internal to the nozzle is a method used in paint sprayers, e.g. the Model 75 paint gun available from the Sharpe Manufacturing Co. of Los Angeles CA. In an attempt to improve the Metriguard high speed ink marking system, the Metriguard system was tested using external mix nozzles available from Spraying Systems Co. of Wheaton IL. These nozzles consisted of two parts, a fluid cap and an air cap. Experiments with fluid cap Model 2050 were run with both air caps Model 67228-45 and Model 64. For these combinations, ink exists from a circular liquid orifice and air exits from an annular gas orifice, the annulus being concentric with the liquid orifice. The nozzle was fit to the Model AM-055-1-12 Angar valve using an intermediate manifold. No combination of ink and air pressures was found that allowed control of the Angar valve and nozzle to start and stop the ink mark in the time required. Further, the structure of the Spray Systems nozzles in common with others of this general type requires them to be disassembled for cleaning, causes them to be more costly than necessary, and makes it physically difficult to mount them closely enough to one another for spray marking a common area. The lumber marking application requires that for best resolution, any one of a set of nozzles can spray the same area at a given time. Any spacing of the nozzles in the direction of motion leads to degradation of marking accuracy along the direction of motion or complication in the control circuitry. Spacing in a direction transverse to the direction of motion leads to problems of aiming the nozzles at a common target area.
The available nozzles of the external mix gas/liquid type all utilized a gas orifice that was symmetrical about the liquid orifice. While attempting to construct a simplified and more compact nozzle of this type, with a symmetrical array of tiny gas holes about a liquid hole in a block of metal to approximate an annular gas orifice, difficulty was encountered in drilling the multiple array of gas holes. Specifically, the tiny drill bit did not survive to the last hole. In frustration, an experiment was performed wherein a nozzle with only one gas orifice adjacent to a liquid orifice was combined with an Angar Model AM-055-1-12 valve and Metriguard Grade Spray Controller. Liquid passageway connections were made from the liquid orifice through the nozzle and valve combination to a pressurized liquid ink source, and a gas passageway connection was made from the gas orifice through the nozzle to a pressurized air source. The surprising result was a nozzle that performs exceptionally well for the ink spray marking application.